1. Field of the Invention
The present invention generally relates to a cleaning agent, and more specifically, it relates to a cleaning agent for cleaning a semiconductor substrate subjected to dry etching. The present invention also relates to a method of fabricating a semiconductor device including a step of cleaning a semiconductor substrate subjected to dry etching with such a cleaning agent.
2. Description of the Prior Art
The pattern of a semiconductor integrated circuit device is increasingly refined in order to improve the speed and the performance of the device. Not only a transistor part influencing the performance of the device but also a capacitor and a multilayer wiling step are refined
In order to form a fine pattern by dry etching employing a resist film as a mask, refinement of the resist pattern and dry etching having higher anisotropy are required. Consequently, a large amount of resist residue adheres onto the fine pattern after ashing for removing the resist film employed for dry etching, and it is difficult to remove the residue with a conventional cleaning liquid.
Further, the target grain size of particles remarkably influencing the, yield of the device is also reduced following refinement of the pattern, and it is necessary to remove smaller particles.
At present, a mixed solution (hereinafter abbreviated as APM) of ammonia and aqueous hydrogen peroxide is widely employed for removing the resist residue and the particles in general. APM, which is an alkaline cleaning liquid, has an effect of finely etching a device material such as a silicon oxide and removing the resist residue and the particles. It has been reported in a learned society or the like that particles hardly adhere to a silicon substrate in an alkaline aqueous solution since the silicon substrate and the particles are negatively charged, and this phenomenon is regarded as the reason for the high cleaning effect of APM.
In a device following a design rule of not more than 0.15 xcexcm, wires for a gate electrode, a bit line and the like are prepared from a metal material such as tungsten or a tungsten alloy having low resistance. Tungsten or an alloy such as tungsten nitride is dissolved due to reaction with aqueous hydrogen peroxide contained in APM. If such a material is exposed, therefore, APM cannot be used. When employing a mixed solution (dilute aqueous ammonia)-of ammonia and water containing no aqueous hydrogen peroxide as a cleaning liquid for preventing dissolution of the material, a silicon material such as silicon forming a semiconductor substrate or polysilicon or amorphous silicon employed as a wire material or an electrode material for a capacitor is dissolved due to reaction with ammonia although dissolution of tungsten is suppressed.
While alkaline cleaning liquids include an aqueous solution (generally used as a developer) of tetramethylammonium hydroxide (TMAH), a water-soluble organic solvent containing organic amine and the like in addition to the aforementioned aqueous ammonia, all these cleaning liquids dissolve silicon and have low cleaning ability.
Thus, there is no proper cleaning liquid removing a resist residue and particles in a state simultaneously exposing tungsten or an alloy such as tungsten nitride and silicon under the present circumstances.
Problems of a conventional method of fabricating a semiconductor device having a step simultaneously exposing tungsten and silicon are now described.
FIG. 2 is a sectional view of a semiconductor device fabricated with a conventional cleaning agent. An isolation insulator film 2 for isolating element regions from each other is provided on a major surface of a semiconductor substrate 1. A gate electrode 6 is formed by stacking a polysilicon film 4 and a tungsten (or tungsten alloy) film 5 on the semiconductor substrate 1 through a gate insulator film 3. An interlayer isolation film 7 is formed on the semiconductor substrate 1 to cover the gate electrode 6. A connection hole 7a exposing the surface of the semiconductor substrate 1 and a connection hole 7b reaching the surface of the gate electrode 6 are formed in the interlayer isolation film 7, and embedded conductive layers 8 consisting of tungsten are embedded in the connection holes 7a and 7b respectively. A bit line 9 consisting of tungsten or a tungsten alloy is provided on the interlayer isolation film 7, to be connected with the embedded conductive layers 8. Interlayer isolation films 10 and 11 are provided on the interlayer isolation film 7, to cover the bit line 9. A connection hole 12a exposing the surface of the semiconductor substrate 1 is formed through the interlayer isolation films 11, 10 and 7. An embedded conductive layer 12 consisting of tungsten is provided to cover the side wall surface and the bottom surface of the connection hole 12a. This embedded conductive layer 12, which must essentially be completely embedded in the connection hole 12a, is not completely embedded in the prior art.
An aluminum wiling layer 13 consisting of aluminum or an aluminum alloy is provided on the interlayer isolation film 11, to be connected with the embedded conductive layer 12. The conventional semiconductor device shown in FIG. 2 is fabricated through steps shown in FIGS. 3 to 15.
The problems of the method of fabricating the conventional semiconductor device shown in FIG. 2 are now described.
Tungsten or a tungsten alloy is used as an electrode material in the conventional semiconductor device, and a gate electrode made of such a material is generally referred to as a metal gate. Tungsten silicide (WSi) was employed as a previous electrode material.
Referring to FIG. 3, the isolation insulator film 2 is formed on the semiconductor substrate 1. Then the surface of the semiconductor substrate 1 is oxidized for forming the gate insulator film 3. The polysilicon film 4 and the tungsten film 5 serving as electrode materials are successively formed on the gate insulator film 3, and a resist pattern 14 is formed thereon.
Referring to FIG. 4, the resist pattern 14 is employed as a mask for performing reactive ion etching (dry etching) on the tungsten film 5 and the polysilicon film 4, thereby forming the gate electrode 6.
Referring to FIGS. 4 and 5, the resist pattern 14 is removed by plasma treatment (referred to as ashing) with a gas containing oxygen. At this time, upwardly extending resist residues 15 adhere to the side walls of the gate electrode 6.
Referring to FIGS. 5 and 6, the resist residues 15 are removed by treatment with a cleaning agent. The cleaning agent is prepared from a mixed solution (dilute aqueous ammonia) of ammonia and water. The dilute aqueous ammonia, having small dissolubility for tungsten, dissolves polysilicon due to reaction with ammonia. Therefore, the polysilicon film 4 forming the gate electrode 6 is transversely etched to narrow the width of the gate electrode 6. Consequently, the electric characteristics of a transistor are disadvantageously deteriorated. When the cleaning agent is prepared from APM, the tungsten film 5 is remarkably dissolved although the polysilicon film 4 is not etched. Thus, APM cannot be used in practice.
While this prior art is described with reference to dry etching employing a resist mask, a similar problem arises also when employing a silicon nitride film as a mask.
A second problem of the method of fabricating the conventional semiconductor device having a step simultaneously exposing tungsten and silicon is now described.
Referring to FIG. 7, the gate electrode 6 is formed on the semiconductor substrate 1 and the interlayer isolation film 7 is formed thereon. A resist pattern 14 is formed on the interlayer isolation film 7.
Referring to FIG. 8, the resist pattern 14 is employed as a mask for performing reactive ion etching (dry etching) on the interlayer isolation film 7, thereby forming contact holes 16 in the interlayer isolation film 7.
Referring to FIGS. 8 and 9, the resist pattern 14 is removed by plasma treatment with a gas containing oxygen. At this time, upwardly extending resist residues 15 adhere to the side walls of the contact holes 16 as shown in FIG. 9.
Referring to FIGS. 9 and 10, the resist residues 15 are removed by treatment with a cleaning agent, which is prepared from dilute aqueous ammonia. With this cleaning agent, silicon forming the semiconductor substrate 1 is isotropically etched to form depressions 17 on the bottoms of the contact holes 16 although the cleaning agent has small dissolubility for the tungsten film 5. When such depressions 17 are formed, the embedded conductive layers 8 are not completely embedded in the contact holes 16 as shown in FIG. 2 but the bit line 9 may be disconnected from the gate electrode 6 or the semiconductor substrate 1. Even if no such disconnection takes place, resistance is disadvantageously increased. If the cleaning agent is prepared from APM, the tungsten film 5 is remarkably dissolved although the semiconductor substrate 1 is not etched. Therefore, APM cannot be used in practice.
While this prior art is described with reference to dry etching employing a resist mask, a similar problem arises also when employing a silicon nitride film as a mask.
A third problem of the prior art is now described.
Referring to FIG. 11, the gate electrode 6, the embedded conductive layers 8 and the bit line 9 are successively formed on the semiconductor substrate 1, and the interlayer isolation films 10 and 11 are formed thereon. A resist pattern 14 is formed on the interlayer isolation film 11.
Referring to FIG. 12, the resist pattern 14 is employed as a mask for forming a contact hole 18 by reactive ion etching (dry etching).
Referring to FIGS. 12 and 13, the resist pattern 14 is removed by plasma treatment with a gas containing oxygen. At this time, resist residues 15 vertically extending upward adheres to the side wall of the contact hole 18.
Referring to FIGS. 13 and 14, the resist residues 15 are removed by treatment with a cleaning agent.
Following higher integration and refinement of the semiconductor device, different types of insulator films are employed in a composite manner for flattening the surface of the substrate. The insulator films are formed by a thermally oxidized silicon film, a silicon oxide film prepared by CVD, a BPSG film containing B, P etc. and the like.
In this example, the interlayer isolation films 7, 10 and 11 are made of different materials. For example, the interlayer isolation film 7 is made of TEOS, the interlayer isolation film 10 is made of BPSG, and the interlayer isolation film 11 is made of TEOS. In formation of the contact hole 18, APM is employed as the cleaning agent since no tungsten is exposed in this portion. The quantity of dissolution (quantity of etching) with APM having dissolubility for a silicon oxide film varies with the type of the silicon oxide film. APM has a larger quantity of etching for the BPSG film forming the interlayer isolation film 10 as compared with the TEOS films forming the interlayer isolation films 7 and 11. Therefore, the interlayer isolation film 10 is transversely etched on the wall surface of the contact hole 18 as shown in FIG. 14, to irregularize the side wall surface of the contact hole 18.
Referring to FIGS. 14 and 15, a tungsten film is formed by CVD in this state for forming the embedded conductive layer 12 consisting of tungsten in the contact hole 18 by dry etching or chemical mechanical polishing (CMP). At this time, the contact hole 18 is not completely filled up with tungsten but defines a cavity as shown in FIG. 15, to result in disconnection in an intermediate stage as the case maybe.
Consequently, the aluminum wiling layer 13 formed on the contact hole 18 as shown in FIG. 2 may not be connected with the semiconductor substrate 1. Even if no such disconnection takes place, the resistance between the aluminum wiling layer 13 and the semiconductor substrate 1 is disadvantageously increased.
When cleaned with APM, the diameter of the contact hole 18 is increased to cause short-circuiting with a wire or the like arranged beside the contact hole 18 as the case may be.
In addition to APM, dilute aqueous ammonia also has an effect of removing resist residues. However, dilute aqueous ammonia cannot be applied since the same etches the semiconductor substrate 1.
While the prior art is described with reference to contact with the semiconductor substrate 1, similar problems arise also with reference to contact with another element such as the gate electrode 6 or the bit line 9.
As hereinabove described, the conventional cleaning agent and the conventional method of fabricating a semiconductor device have the problems of dissolving tungsten, an alloy such as tungsten nitride or silicon and causing difference between quantities of etching of different types of oxide films, to consequently deteriorate the characteristics of the semiconductor device by disconnecting the wire and the embedded conductive layer and increasing the resistance.
Accordingly, an object of the present invention is to provide a cleaning agent for a semiconductor device, which is so improved as not to disconnect a wire and an embedded conductive layer.
Another object of the present invention is to provide a cleaning agent for a semiconductor device, which is so improved as not to increase the resistance of a wire or an embedded layer.
Still another object of the present invention is to provide a method of fabricating a semiconductor device, which is so improved as not to disconnect a wire or an embedded layer.
A further object of the present invention is to provide a method of fabricating a semiconductor device, which is so improved as not to increase the resistance of a wire or an embedded layer.
A cleaning agent for a semiconductor device according to a first aspect of the present invention contains a hydroxide, water and a compound expressed in the following general formula (I) and/or the following general formula (II):
HOxe2x80x94((EO)xxe2x80x94(PO)y))zxe2x80x94Hxe2x80x83xe2x80x83(I)
where EO represents an oxyethylene group; PO represents an oxypropylene group, x and y represent integers satisfying x/(x+y)=0.05 to 0.4, and z represents a positive integer.
Rxe2x80x94[(EO)xxe2x80x94(PO)y)zxe2x80x94H]mxe2x80x83xe2x80x83(II)
where EO, PO, x, y and z are defined identically to those in the general formula (I), R represents a residue of alcohol or amine excluding a hydroxyl group or a hydrogen atom of an amino group, and m represents an integer of at least 1.
The oxyethylene group is expressed as xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94, and the oxypropylene group is expressed as xe2x80x94CH(CH3)xe2x80x94CH2xe2x80x94Oxe2x80x94 or as xe2x80x94CH2xe2x80x94CH(CH3)xe2x80x94Oxe2x80x94.
Dissolubility in preparation of the cleaning agent is insufficient if the value of x/(x+y) is less than 0.05, while defoamability of the cleaning agent is insufficient if the value is greater than 0.4.
The part expressed as (EO)xxe2x88x92(PO)y)z in each of the general formulas (I) and (II) may be a block copolymer, a random copolymer or a blocky random copolymer, and the block copolymer is preferable among these.
Alcohol forming the aforementioned R is prepared from monohydric alcohol such as 2-ethylhexyl alcohol, lauryl alcohol, cetyl alcohol, oleyl alcohol, stearyl alcohol, tridecyl alcohol, tallow alcohol or coconut oil alcohol or polyhydric alcohol such as ethylene glycol, propylene glycol, 1, 3-propanediol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, 1,4-butanediol, 2-methyl-1,2-propanediol, 2-methyl-1,3-propanediol, glycerin, trimethylol ethane, trimethylol propane, pentaerythritol or sorbitol, and amine is prepared from methylene diamine or propylene diamine.
The cleaning agent according to the first aspect of the present invention hardly dissolves tungsten or an alloy such as tungsten nitride, silicon and an insulator film, and exhibits the same quantity of etching for different types of insulator films. Consequently, the cleaning agent attains such an effect that the width of a gate electrode is not narrowed.
Preferably, the aforementioned hydroxide is ammonium hydroxide.
In this case, the amount of impurities contained in a solution is so small that no impurities remain on the surface of a semiconductor substrate since ammonium hydroxide is employed as the hydroxide. Preferably, the aforementioned hydroxide is selected from a group consisting of tetramethylammonium hydroxide, a hydroxide of potassium and a hydroxide of sodium.
The concentration of the hydroxide contained in the aforementioned cleaning agent is preferably 0.01 to 31 percent by weight, and more preferably 0.1 to 3 percent by weight. A sufficient cleaning effect cannot be attained if the concentration of the hydroxide is excessively low, while the quantity of etching for silicon is increased if the concentration of the hydroxide is excessively high. Therefore, the concentration of the hydroxide is preferably in the range of 0.01 percent by weight to 31 percent by weight.
Preferably, the mean molecular weight of the total of the oxypropylene group in the compound expressed in the general formula (I) and/or (II) is 500 to 5000.
The cleaning effect is insufficient if the mean molecular weight is too small, while dissolubility in preparation is insufficient if the mean molecular weight is too large.
Preferably, the weight ratio of the compound expressed in the general formula (I) and/or (II) to the hydroxide is (0.3xc3x9710xe2x88x924 to 1):1.
The quantity of etching for silicon is increased if the ratio of the copolymer is too small, while defoamability is insufficient if the ratio of the copolymer is too large.
Preferably, the pH of the aforementioned cleaning agent is rendered at least 8.
Preferably, the cleaning agent further contains not more than 1 percent by weight of hydrogen peroxide.
The quantity of etching for tungsten, which is increased if the content of hydrogen peroxide is large, can be reduced to a proper level if the content of hydrogen peroxide is not more than 1 percent by weight, while the quantity of etching for silicon can be further reduced due to mixing with hydrogen peroxide.
A method of fabricating a semiconductor device according to a second aspect of the present invention comprises a first step of preparing a semiconductor substrate completely subjected to dry etching and a second step of cleaning the surface of the semiconductor substrate with a cleaning agent containing a hydroxide, water and a compound expressed in the following general formula a) and/or the following general formula (II):
HOxe2x80x94((EO)xxe2x80x94(PO)y)zxe2x80x94Hxe2x80x83xe2x80x83(I)
where EO represents an oxyethylene group, PO represents an oxypropylene group, x and y represent integers satisfying x/(x+y) 0.05 to 0.4, and z represents a positive integer.
Rxe2x80x94[(EO)xxe2x80x94(PO)y)zxe2x80x94H]mxe2x80x83xe2x80x83(II)
where EO, PO, x, y and z are defined identically to those in the general formula (I), R represents a residue of alcohol or amine excluding a hydroxyl group or a hydrogen atom of an amino group, and m represents an integer of at least 1.
The conventional cleaning agent has such a problem that a polysilicon film forming a gate electrode is transversely etched to narrow the width of the gate electrode and deteriorate the electrical characteristics of a transistor. When employing the aforementioned cleaning agent, however, the width of the gate electrode is not narrowed since the polysilicon film is not etched.
Preferably, the aforementioned first step includes steps of performing the aforementioned dry etching with a resist pattern and removing the resist pattern by ashing.
In this case, a resist residue adhering onto a fine pattern can be efficiently removed.
Preferably, the aforementioned first step includes a step of exposing a metal film containing tungsten and/or a silicon material on the aforementioned semiconductor substrate by the aforementioned dry etching.
In this case, a resist residue and particles can be efficiently removed in such a state that tungsten or an alloy such as tungsten nitride and silicon are simultaneously exposed.
Preferably, the aforementioned first step includes steps of forming a wiring pattern containing polysilicon and tungsten on the aforementioned semiconductor substrate by the aforementioned dry etching employing a resist pattern and removing the resist pattern.
In this case, a polysilicon film forming a gate electrode is not transversely etched. Thus, the width of the gate electrode is not narrowed.
Preferably, the aforementioned first step includes steps of forming a wiling pattern containing tungsten on the aforementioned semiconductor substrate, forming an insulator film on the wiling pattern, forming a connection hole in the insulator film by the aforementioned dry etching employing a resist pattern, and removing the resist pattern.
In this case, no depression is formed on the bottom of the connection hole. Thus, an embedded conductive layer is completely embedded in the connection hole.
Preferably, the aforementioned first step includes steps of forming at least two types of silicon oxide insulator films on the aforementioned semiconductor substrate and performing the aforementioned dry etching on the two types of silicon oxide insulator films.
In this case, the side wall surface of a contact hole is not irregularized. Thus, tungsten or the like is completely embedded in the contact hole.
Preferably, the aforementioned cleaning is performed by setting the liquid temperature of the aforementioned cleaning agent to 20xc2x0 C. to 50xc2x0 C. and dipping the aforementioned semiconductor substrate in the cleaning agent.
Removability for a resist residue is reduced if the liquid temperature is not more than 20xc2x0 C., while dissolubility for silicon or an insulator film is increased if the liquid temperature is in excess of 65xc2x0 C. Therefore, the liquid temperature of the aforementioned cleaning agent to 20xc2x0 C. to 65xc2x0 C., for dipping the semiconductor substrate.
Preferably, the aforementioned cleaning is performed by setting the liquid temperature of the aforementioned cleaning agent to 20xc2x0 C. to 65xc2x0 C. and spraying the cleaning agent to the semiconductor substrate.
Removability for a resist residue is reduced if the liquid temperature is not more than 20xc2x0 C., while dissolubility for silicon or an insulator film is increased if the liquid temperature is in excess of 65xc2x0 C. Therefore, the cleaning is desirably performed by spraying at the liquid temperature in the range of 20xc2x0 C. to 65xc2x0 C.
Preferably, the aforementioned cleaning is performed by introducing ultrasonic waves into the aforementioned semiconductor substrate.
If the cleaning is performed for removing particles or the like, the cleaning effect is further improved by employing physical cleaning with ultrasonic waves or the like.